FIG. 1 shows the basic configuration of an optical disk device.
Optical disk 10 is a toroidal circular disk with a central hole. Arrays of bits are arranged in a concentric (specifically, spiral) pit pattern to form tracks on the signal recording side 10a (underside as shown in FIG. 1) of the disk. During playback, spindle motor 12 drives optical disk 10 to rotate at a prescribed linear velocity using, for example, the CLV (Constant Linear Velocity) method.
A light pick-up 14 that can move in the radial direction of the disk is set opposite the signal recording side 10a of optical disk 10. Light pick-up 14 focuses and radiates a laser beam LB onto the signal recording side 10a of optical disk 10, detects the light beam reflected from signal recording side 10a, performs photoelectric transformation, and generates an electric signal having a waveform corresponding to the embossment pattern of the pit arrays. The electric signal output from light pick-up 14 is sent to an RF amplifier 16. RF amplifier 16 generates a reproduced data signal SM, as well as a tracking error signal TE and a focus error signal FE. The reproduced data signal SM output from RF amplifier 16 is input to a data signal processing unit (not shown in the figure), where decoding, error correction, or other signal processing is performed. A servo error signal, such as the tracking error signal TE or focus error signal FE, output from RF amplifier 16 is sent to a pick-up servo 18. Pick-up servo 18 performs tracking control for positioning or tracking the beam spot SP of laser beam LB on the track (pit array) and also performs focusing control for focusing beam spot SP to the size of the pit.
FIG. 2 shows an example of the light detector incorporated as the photoelectric transformation part in light pick-up 14. The light detector is a four-division type used for a push-pull system. Four photoelectric transformation units or light receiving regions A, B, C, D made of electrodes, for example, are arranged in adjoining abutting four quadrant relationship. The beam LB′ of light returning or reflected from the signal recording side 10a of optical disk 10 is focused and made incident onto the light receiving regions A, B, C, D of the light detector through optical lenses or other detecting optical systems. Electric signals (referred to as RF signals hereinafter) SA, SB, SC, SD are generated corresponding to the quantity or intensity of light received in each of light receiving regions A, B, C, D. The RF signals are typically voltage signals obtained after I–V conversion performed in pick-up 14. A reference voltage Vc applied as an external bias voltage to the pick-up is used as the reference level. FIGS. 3A–3D schematically illustrate the waveforms of RF signals SA, SB, SC, and SD.
The RF signals SA and SD obtained from light receiving regions A and D on the left side of the boundary that is the central line in parallel with the track direction have the same phase. The RF signals SB and SC obtained from the light receiving regions B and C on the right side of the same boundary also have the same phase. The RF signals SA, SD have opposite phases from RF signals SB, SC.
When the beam spot SP of laser beam LB on signal recording side 10a of optical disk 10 is positioned on the track center, that is, when it is on track, the light beam LB′ reflected from signal recording side 10a is incident onto the central part of the light receiving regions A, B, C, D of the light detector. The level of the RF signals SA and SD obtained from the light receiving regions A and D on the left side is almost the same as that of the RF signals SB and SC obtained from the light receiving regions B and C on the right side.
However, when beam spot SP shifts from the center of the track in the radial direction, the reflected light beam LB′ is incident onto a position that deviates to the left or right from the central part of the light receiving regions A, B, C, D of the light detector. The level of the RF signals SA and SD obtained from the light receiving regions A and D on the left side becomes different from that of the RF signals SB and SC obtained from the light receiving regions B and C on the right side. For example, if beam spot SP shifts inward in the radial direction, the focusing position of the reflected light beam LB′ is offset to the left side of light receiving regions A, B, C, D. The level of the RF signals SA and SD obtained from the light receiving regions A and D on the left side becomes higher than that when the beam spot is on track. On the other hand, the level of the RF signals SB and SC on the side of right-side light receiving regions B and C becomes lower than that when the beam spot is on track. If beam spot SP shifts outward in the radial direction, the focusing position of reflected light beam LB′ is offset to the right side of the light receiving regions A, B, C, D. The level of the RF signals SB and SC obtained from the right-side light receiving regions B and C becomes higher than that when the beam spot is on track, while the level of the RF signals SA and SD obtained from the left-side light receiving regions A and D becomes lower than that when the beam spot is on track.
FIG. 4 (prior art) shows the basic configuration of a conventional tracking error detecting circuit in a push-pull system. The tracking error detecting circuit has a pair of adders 200 and 202, a subtracter 204, and a low-pass filter 206. Adder 200 calculates the sum (SA+SD) of the RF signals SA and SD obtained from the left-side light receiving regions A and D of the light detector (FIG. 2), while the other adder 202 calculates the sum (SB+SC) of the RF signals SB and SC obtained from the right-side light receiving regions B and C. Subtracter 204 calculates the difference {(SA+SD)−(SB+SC)} of the two signals. Low-pass filter 206 eliminates the high-frequency component, that is, the RF signal component from the difference signal output from subtracter 204 and outputs tracking error signal TE.
FIG. 5 (prior art) shows the basic configuration of a conventional focus error detecting circuit using the astigmatism method. The focus error detecting circuit has a pair of adders 208 and 210, as well as a subtracter circuit 212. Adder 208 calculates the sum (SA+SC) of the RF signals SA and SC obtained from light receiving regions A and C positioned on one of the diagonals of the light detector (FIG. 2). The other adder 210 calculates the sum (SB+SD) of the RF signals SB and SD obtained from the light receiving regions B and D positioned on the other diagonal. Subtracter 212 calculates the difference {(SA+SC)−(SB+SD)} of the two signals. The difference signal is taken as the focus error signal FE. The output signal FE of subtracter 212 can also pass through an amplifier or a low-pass filter (not shown in the figure).
According to the astigmatism method, when the objective lens in light pick-up 14 is too close to optical disk 10, the spot of the reflected light beam focused onto the light receiving region A, B, C, D of the light detector becomes an elliptic shape that is stretched on the side of light receiving regions A and C and is compressed on the side of light receiving regions B and D, as shown by broken line LBa in FIG. 6. On the other hand, when the objective lens is too far away from optical disk 10, the spot of the reflected light beam focused in the light receiving region A, B, C, D of the light detector becomes an elliptic shape that is stretched on the side of light receiving regions B and D and is compressed on the side of light receiving regions A and C, as shown by dot-dashed line LBb in FIG. 6.
In an optical disk device, the accuracy of a servo error signal, such as the tracking error signal TE or the focus error signal FE, controls the accuracy of the servo. For example, for a tracking servo, when the light pick-up is jump-shifted from the track during the current tracking to another track, if the accuracy of the tracking error signal is low, a long time will be required from applying the tracking servo near the targeted track until reaching the state of on-track, or it is difficult to correctly perform an on-track operation.
FIGS. 7A and 7B schematically illustrate the waveforms of RF signals SA, SB, SC, SD obtained from light receiving regions A, B, C, D of the light detector (FIG. 2) during tracking. The reason for the sinusoidal level change Ste at the bottom of the waveform of each RF signal is that the beam spot SP of the laser beam LB radiated from light pick-up 14 onto the signal recording side 10a of optical disk 10 crosses alternately between the track part (pit) and the intertrack mirror part (flat part where there is no pit) in the radial direction at a certain velocity. In other words, the reason is that when the beam spot SP crosses the track part, the intensity modulation caused by the pits or the diffraction is maximized (as a result, the light intensity of the reflected light beam is minimized), and when the beam spot SP crosses the mirror part, the intensity modulation is minimized (as a result, the light intensity of the reflected light beam is maximized). Usually, even when the beam spot SP is at the center of a mirror part, it partially overlaps the adjacent track, and intensity modulation occurs in the overlapped part. As a result, for the RF signal, the maximum point of the bottom level is not as high as the top level, and there is a high-frequency modulation component even near the maximum point.
In a conventional tracking error detecting circuit (FIG. 4), operation circuits 200, 202, 204 perform the required operation (push-pull operation) for the RF signals SA, SB, SC, SD obtained from the light receiving regions A, B, C, D of the light detector, respectively. In this way, tracking error signal TE that indicates the positional error of beam spot SP in the radial direction can be obtained. However, whether in low-speed or high-speed playback, as a result of passing through the low-pass filter, the tracking signal TE is attenuated significantly (by about one-half), leading to deterioration in the SN (signal-to-noise) characteristic (see FIGS. 8 and 9). The reason for this is that the top envelope component of the RF signal is almost flat. Also, in the case of low-speed playback, the frequency of the RF signal is relatively close to that of the tracking error signal. It is difficult to separate the two signals using a low-pass filter. The RF signal component tends to be left over in the tracking error signal TE as shown in FIG. 8.
In general optical disks, such as a CD (Compact Disc) and a DVD (Digital Versatile Disk), EFM (eight to fourteen modulation) or EFM Plus is usually used as the modulation method for the data recorded on the tracks. The length of the pits is limited to the range of 3T–11T (T is the length of one bit) in order to satisfy the [2, 10] RLL (Run-Length Limited) code condition, that is, the length of “0” is in the range of 2–11. Therefore, for example, in a CD, the frequency of the channel clock is 4.32 MHz at double speed. However, the frequency of the RF signal reaches the highest level of 720 kHz in the case of 3T/3T. It is as low as 196 kHz in the case of 11T/11T. In order to perform tracking servo correctly, such an RF signal component in the tracking error signal is desired to be so small that it can be ignored.
In the conventional tracking error detecting circuit, however, it is difficult to separate or cut the RF signal component with a frequency of about 196 kHz from the tracking error signal TE obtained from the tracking modulation component Ste that usually has a frequency of tens of kHz by passing the signal through low-pass filter 106.
Also, in order to match the input range of the AD converter (not shown in the figure) in a later stage during conversion to digital signals, the amplitude of the tracking error signal TE is increased by using operation circuits 200, 202, 204 or a special gain control amplifier (not shown in the figure). However, since the RF signal component is also amplified, the gain cannot be increased sufficiently. In addition, the accuracy of the digital signal becomes low due to mixing of the RF signal component.
On the other hand, during high-speed playback performed at 30-fold speed for a CD or 6-fold speed for a DVD, the band of the RF signal becomes 10 MHz or higher. When the band of operation circuits 200, 202, 204 is only several MHz, these operation circuits act as low-pass filters. They not only attenuate the RF signal component but also reduce the tracking error signal TE by half to its original form, that is, to the tracking modulation component Ste. As a result, SN becomes as low as 6 dB.
In a conventional focus error detecting circuit (FIG. 5), the accuracy of the focus error signal and SN are also low. Other servo error detecting systems using light detectors other than the four-division type also have the same problem.
There is a need to solve the problem of the conventional technology by providing a servo error detecting device for an optical disk that can generate error signals for servo, from which the high-frequency component can be effectively cut off.
There is also a need to provide a servo error detecting device for an optical disk that can generate error signals for a servo with a high SN ratio.